I had a turn signal out on my 2007 Toyota 4Runner Limited V8 and thought this would be a great time to upgrade to LED turn signals. I swapped in these:
- Front: https://www.amazon.com/gp/product/B07VDNSLLK
- Rear: https://www.amazon.com/gp/product/B0755F9G1P
They seem well built and are significantly brighter than the stock incandescent bulbs.
However, due to the lower current draw of the LED lights, the flasher module in the vehicle thinks the bulb is out and does a fast blink or “hyper flash”. A common fix is to simply put a high current resistor in parallel with the LED bulbs to increase the current draw but this didn’t seem like an ideal solution for a few reasons:
- More wiring & components = More points of failure
- More janky stuff hanging around under the hood
- Wastes the additional power as heat (not a big deal really, but inelegant)
- Probably prevents detection and hyper flashing if the LED bulb blows
I did find that aftermarket flasher modules exist that are designed to work with LEDs but as my 4Runner has Daytime Running Lights (DRL), it uses the “12-pin” flasher module which has no LED version available as a replacement.
Luckily I did find a simple mod to the existing module that fixes the problem. The module contains a current sensing shunt (looks like a wire jumper). By swapping out the shunt with a higher resistance, the target current range that the module identifies as a working bulb can be decreased.
In the stock configuration, the nominal voltage drop on the shunt for one side flashing is 75mV  (twice this for hazards, i.e. both sides flashing). With the stock bulbs this corresponds to a stock shunt resistance of about 20mΩ.
My new LED bulbs are rated at 6W for the front and 5.5W for the rear, for 11.5W total. This corresponds to a current draw of 0.83A @ 13.8V. To produce the same 0.075V drop in the shunt, I would need a resistance of 63mΩ.
I decided to try out a 70mΩ resistor as a more common value. To calculate the power dissipated in the shunt itself we can use P = IV = 0.83 * 0.075 = 0.062 W. I got some completely overkill 3W ones just in case:
Flasher Module Location
Its a little fiddly to remove as it locks in place onto the top of the fuse box. It can be released by inserting a small screwdriver into the latch from the same end as the connector to release the tab.
Replacing the Shunt Resistor
The case of the module can be released with 4 tabs around the connector faceplate. Its fiddly but be patient and don’t force it, I used various shims to hold each tab open after releasing them one at a time.
The shunt resistor is the metal loop marked “RS”. Simply remove by desoldering one end at a time. You can see the underside of the pads in the photo near C4. After this, I cleaned up the pads with some solder wick to make installing the new resistor easier.
New resistor installed (0.070R).
After reinstalling I verified that the lights were flashing at the normal rate on both sides, and with the hazards. I then tested to see that hyper-flashing would return if one of the bulbs was disconnected, which it did.
Overall this was a simple fix that is robust, preserves hyper-flashing functionality to detect blown bulbs, and doesn’t waste excessive power.